fix: cascade-delete all orphaned objects in PreviewHost cleanup

[teradyl]

Summary

removeExisting only deleted the first matching PreviewHost native target and a single root-group child. Everything else attached to the target was orphaned in the pbxproj on every preview run, and because each leak left another PreviewHost target / group, the next run's .first(where:) only peeled one off — so projects accumulated hundreds of lines of cruft and broken file references pointing at deleted .preview-host-XXXXX/ temp dirs.

What changed

removeExisting now:

• Iterates over all PreviewHost native targets (not just .first) and cascade-deletes:
  • build phases + their build files,
  • configuration list + individual configurations,
  • target dependencies + container item proxies,
  • SPM package product dependencies,
  • the PreviewHost.app product reference (also removing it from the Products group),
  • the target's slot in project.targets and the target object itself.
• Recursively scans for all PreviewHost PBXGroups and deletes each child file ref before removing the group.
• Final pass scrubs orphan PreviewHost.app refs in the Products group left behind by earlier broken cleanups.

Test plan

• New regression test testCleanupLeavesNoOrphanedObjects runs inject + cleanup three times on the same project and asserts no PreviewHost-related target, group, file ref, or build configuration survives any cleanup pass.
• All existing ProjectInjectorTests pass (cleanup, idempotent injection, scheme creation, etc.).
• Verified end-to-end on a real project: after the patch, repeated /preview runs add 0 lines to project.pbxproj (and actually scrub pre-existing orphan refs from prior buggy runs).